There may be a scientific way to do it, but if so I didn't bother.

I sent one character to one set of levers to the west and another to levers to the south and pulled levers at random (occasionally jumping up and down and gibbering) until I stumbled on the combination that opened the door that is blocking you. You need to use those levers - there is no key.

Reality is for those who cannot cope with fantasy. Fortunately, I can cope quite well.